Abstract : The purpose of this thesis is to investigate the performance of the Digital Excision Temporal Filter (DETF) to reject narrowband jammers used against the Global Positioning System (GPS). The DETF takes the Fast Fourier Transform (FFT) of the GPS signal and excises any FFT bins that are above a preselected threshold level. Then the excised signal is Inverse Fourier Transformed and fed to the GPS receiver. Several jammer types are simulated including Continuous Wave (CW), Pulse CW, Swept CW, Narrowband Spot Noise, and Wideband Barrage Noise jammers. Cases are also simulated using all but the Wideband Barrage Noise jammer at one time. The DETF can effectively reject all of the types of jammers simulated except for the Wideband Barrage Noise jammer. The DETF degrades the GPS system performance in the presence of the Wideband Barrage Noise jammer. In an actual DETF implementation, the excision threshold should be set from six to nine dB above the excision cutoff level, where the excision cutoff level is equal to the GPS signal strength plus receiver thermal noise level.